This invention relates to an electrographic writing head assembly. More particularly, it relates to an amorphous silicon electrographic writing head assembly having a protective cover.
Amorphous silicon, a-Si, technology has found numerous applications because of its low cost and compatibility with low temperature glass substrates. Circuits regularly fabricated with linear dimensions in excess of 30 cm enable the fabrication of large area electronic circuits. Printing systems based upon Ionography and Electrography have also been demonstrated with a-Si. For instance, U.S. Pat. No. 4,466,020 to O'Connell describes an integrated imaging bar having both an array of photosensitive elements and an array of associated marking elements. As will become apparent, the electrographic writing system of the present invention is a good candidate for using large area technology and a-Si fabrication techniques. However, problems can exist when incorporating an a-Si device into an electrographic writing head assembly. The problems and solutions will be discussed below.
In general, the electrographic recording process, for which this invention is particularly applicable, includes the steps of forming an electrostatic latent image upon a recording medium and subsequently making the latent image visible. FIG. 1 shows a typical electrographic plotting system 10 including medium roll 12, medium 14, writing head assembly 16, toning station 18 and drive roller 20. Recording medium 14 is provided in web form and is payed from roll 12 as it is driven by drive roller 20. Medium 14 has a dielectric surface which comes in approximate contact with writing head assembly 16 and a conductive surface (not shown) which is opposite the dielectric surface. Medium 14 may be a coated paper, a polyester based transparent film, or other suitable material on which an electrostatic latent image is formed. Writing takes place in electrography when the potential difference between writing electrodes 28 (also referred to as styli or nibs) of writing head assembly 16 and a biased complementary electrode (not shown) on medium 14 is sufficient to break down the air gap therebetween forming an electrostatic latent image.
As shown in FIG. 1, writing head assembly 16 includes substrate 22 on which an array of thin film transistor elements (not shown) and electrodes 28 are formed. Substrate 22 may be glass with the thin film elements being fabricated thereon utilizing a-Si technology. For mechanical strength and protection, substrate 22 is packaged by being sandwiched between protective insulating overcoatings 24, 26. At the edge of head 16, in contact with the medium, the ends of the conductive electrodes 28 are exposed and are maintained slightly spaced from the surface of the recording medium by an air gap through which selective ionizing electrical discharge takes place forming the electrostatic latent image, as discussed above.
Subsequently, the latent image is made visible during the development step by applying liquid or dry toner to the recording medium using toning station 18. The recording medium is contacted by a thin film of developer material out of which the toner particles are electrostatically attracted to the regions of electrostatic charge on the medium defining the electrostatic latent image hence forming a visible image.
A typical example of thin film transistor elements and electrodes included as part of an electrographic writing array, manufacturable by thin film fabrication techniques, is fully disclosed in U.S. Pat. No. 4,588,997 to Tuan et al. which is hereby incorporated by reference. The thin film components described in Tuan et al. include a linear array of several thousand styli (electrodes) or nibs, a high voltage thin film transistor (HVTFT), and a low voltage thin film transistor (LVTFT). The styli are for generating sequential raster lines of information by means of high voltage electrical discharges across a minute air gap to a conductive electrode. In order to drive selected styli in the array, a multiplexing scheme is used wherein the charge on each stylus is controlled by a low voltage thin film pass transistor which selectively charges and discharges the gate of a thin film high voltage transistor for switching the HVTFT. This scheme allows each stylus to maintain its imposed charge, for substantially a line time, between charges and discharges.
However, it has been shown that the high voltage thin film transistor HVTFT described in Tuan et al. suffers from a characteristic drift in voltage over time causing it to be unstable. U.S. Pat. No. 4,998,146 to Hack discusses this characteristic drift in voltage and offers a solution comprising extending the gate over the channel of the high voltage transistor. U.S. Pat. No. 4,984,040 to Yap also addresses the issue of voltage drift in high voltage thin film transistors and offers a solution by adding a second gate to the high voltage transistor. U.S. Pat. No. 4,998,146 to Hack and U.S. Pat. No. 4,984,040 to Yap, assigned to the same assignee as the present application, are hereby incorporated by reference.
Even though the problem of voltage drift in the high voltage thin film transistors of prior electrographic writing arrays has been addressed, it has been discovered that the problem of voltage drift reappears when constructing an electrographic writing head assembly utilizing an array such as that described in Tuan et al., even when incorporating the improvements suggested by Hack and Yap. In particular, the packaging of the adhesive, used over high voltage transistor circuitry on the array which causes the voltage drift to reappear. Therefore, in light of the above discussion, it would be highly desirable to have an electrographic writing head assembly which utilizes an array having thin film devices in which the voltage drift problem in the high voltage transistor is eliminated.